A method for fixing an image recorded on a recording medium is roughly divided into permeation, drying and curing processes.
The permeation is a process wherein a solvent component in colorant (such as ink or paint) is absorbed into the recording medium. In this case, since the fluidity of the colorant is solely changed, the fastness of the image is not basically improved. While the high image fastness would be obtained, of course, as the time has passed if a binder component or a cross-linker is contained in the colorant, it is not obtainable solely by the permeation, directly after the formation of the image.
The drying is a process wherein the solvent component in the colorant is removed from the recording medium. Also in this case, since the solvent component is solely removed from the colorant, it is difficult to obtain a high fastness directly after the formation of the image as in the case of permeation. In this case, however, if the heating which is the most popular drying means is adopted together with the curing described hereinafter, there may be cases wherein the high fastness is obtainable.
The curing is a process for changing the binder component in the colorant to be fixed. Usually, such change is the polymerization of binder resin by the apply of energy thereto, whereby the high fastness is obtainable even directly after the image formation. The energy to be applied may generally be heat, ultraviolet ray (UV) and electron beam (EB), which have merits and demerits, respectively.
Heating is the most representative means used for the curing, and has merits wherein it is capable of causing the curing reaction simultaneously with or continuously to the removal of solvent from material containing the latter, or material is selectable from a wider range. On the contrary, it has demerits wherein usable recording medium is restricted because of the damage caused by heat or the dimensional stability of the recording medium becomes inferior due to the thermal expansion.
The curing process by the irradiation of ultraviolet ray as disclosed in Japanese Patent Laid-Open No. 2003-145714 or others is more advantageous than the curing by the heating since it is accompanied with no heat. However, the colorant may be restricted since an ultraviolet ray curing ink must be used, for example. Also, if an ink layer on the recording medium is too thick or has a deep color portion having a high light-absorbent efficiency, the ultraviolet ray is difficult to reach a bottom of the ink layer, whereby the interior of the image is left uncured while the surface of the image is cured.
On the other hand, since the curing process by the irradiation of electron ray disclosed in Japanese Patent Laid-Open No. 10-100379 (1998) or others is excellent in permeability, it is possible to preferably carried out the curing even if the ink layer on the recording medium is thick or has a deep color. Since secondary X-rays are irradiated, however, it is necessary to provide a strict shielding structure for the purpose of the operator's safety.